Coronary artery disease and perioperative myocardial infarction

1

What are the known risk factors for the development of coronary artery disease?

Age, male gender, and positive family history (first-degree relative with coronary artery disease [CAD], male < 55 years or female < 65 years) are risk factors that cannot be modified. Smoking, hypertension, diet, dyslipidemia, physical inactivity, obesity, and diabetes mellitus are modifiable risk factors.

2

Describe the normal coronary blood flow.

The resting coronary blood flow (CBF) averages about 225 mL/min, which is 4% to 5% of the total cardiac output (CO) in normal adults. The CBF increases up to four fold to supply the extra nutrients needed by the heart at maximum coronary vasodilation, also known as hyperemia . The driving force for CBF is determined by the coronary perfusion pressure (CPP), which is the gradient between the aortic pressure (P _aorta ) and the ventricular pressure (P _ventricle ). There are phasic changes in CBF to the left ventricle, during systole and diastole. The CBF to the left ventricle significantly decreases in systole as the left ventricular pressure equals or exceeds the pressure in the aorta, causing the CPP to essentially equal zero. It is important to appreciate that in the case of left ventricular outflow obstruction (i.e., hypertrophic cardiomyopathy or aortic stenosis), the left ventricular intracavitary pressure may significantly exceed that of the aortic pressure during systole. In diastole, the cardiac muscle fibers relax, allowing blood to flow through the left ventricular capillaries. Further, the aortic valve closes, which creates a natural pressure gradient between the higher aortic pressure and the lower left ventricular diastolic pressure. This increases the CPP, which is the main driving force for CBF.

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What happens to CBF during systole and diastole?

The left heart is only perfused during diastole, as the aortic pressure (P _aorta ) is greater than the ventricular pressure (P _ventricle ). Therefore it is important to avoid tachycardia to maintain coronary perfusion to the left heart. The right heart, however, is perfused during systole and diastole, as the aortic pressures are generally higher than the right ventricular pressure in both systole and diastole.

4

What is the equation for coronary perfusion pressure?

Coronary perfusion pressure (CPP) can be explained by the following equation:

CPP , coronary perfusion pressure ; P _aorta , aortic pressure ; P _ventricle , intraventricular pressure

Although this equation is true for the right heart for both systole and diastole, the left heart is only perfused during diastole where the equation can be simplified to the following:

CPP , coronary perfusion pressure ; dBP , aortic diastolic blood pressure ; LVEDP , left ventricular end-diastolic pressure .

5

Describe the coronary anatomy.

The right coronary artery system is dominant in that it supplies blood to the inferior left ventricular wall via the posterior descending artery (PDA) in about 85% of people. The right coronary also supplies the sinoatrial node (SA), atrioventricular (AV) node, and right ventricle. Right-coronary artery occlusion can result in bradycardia, heart block, myocardial infarction (MI) of the right ventricle and/or inferior left ventricular wall.

The left main coronary artery (LMCA) gives rise to the left circumflex artery (LCx) and the left anterior descending (LAD) artery. The LAD and the PDA provide blood flow to the interventricular septum via septal branches. The LAD supplies the anterior left ventricular wall directly, and the lateral left ventricular wall via diagonal branches. When the circumflex gives rise to the PDA, the circulation is termed left dominant , where the left coronary circulation then supplies the entire septum and the AV node. In 40% of patients, the circumflex supplies the SA node. Because the LMCA and the LAD supplant much of the blood flow to the left ventricle, occlusion of these arteries, with resultant ischemia, can result in severely depressed left ventricular function, and in the prognostically worst case, cardiogenic shock.

6

Describe the determinants of myocardial oxygen supply and their relationship.

Oxygen delivery to the myocardium is the product of CBF and the oxygen content of arterial blood (CaO ₂ ):

Recall, that oxygen content (CaO ₂ ) is determined by the following:

CBF is governed with the same relationship as Ohm’s law, I = △ V/R, where “ΔV” represents the coronary perfusion pressure and “I” represents CBF:

P _aorta , aortic root pressure ; P _ventricle , ventricular chamber pressure ; CVR, coronary vascular resistance

Therefore myocardial oxygen supply can be rewritten as the following:

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How can you increase myocardial oxygen supply and delivery?

From the earlier equation, the myocardial oxygen supply can be increased by the following:

• 1)

  Increase hemoglobin concentration [Hg] by transfusing red blood cells

• 2)

  Maintain an oxygen saturation (SaO ₂ ) of 100% with supplemental oxygen

• 3)

  Maintain an adequate coronary perfusion pressure (P _aorta − P _ventricle ) with vasopressors (i.e., phenylephrine) to increase P _aorta

• 4)

  Reduce ventricular pressure (P _ventricle ) with diuretics and/or venodilators (e.g., nitroglycerin)

• 5)

  Avoid tachycardia as ventricular pressure (P _ventricle ) increases during systole decreasing CBF

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How can this be used to understand myocardial ischemia? How does this pertain to coronary artery disease, aortic stenosis, and right heart failure caused by a pulmonary embolism?

In referring to the earlier equations for CBF, anything that decreases aortic blood pressure, increases ventricular pressure, increases coronary resistance (e.g., coronary stenosis or thrombosis), or decreases oxygen delivery (e.g., anemia) can cause myocardial ischemia.

In patients with CAD, it is important to avoid tachycardia, as again the left heart is only perfused during diastole. Further, any medical condition associated with an excessively high ventricular filling pressure (e.g., congestive heart failure, end-stage renal disease, aortic stenosis, pulmonary embolism) can decrease coronary perfusion. Recall, if P _ventricle increases, then coronary perfusion pressure decreases, where CPP = P _aorta − P _ventricle . Therefore management of patients with these conditions include strategies to optimize coronary perfusion (e.g., diuretics or venodilators to decrease P _ventricle or vasopressors to increase P _aorta ).

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